Methane fluxes on agricultural and forested boreal organic soils

Abstract. Annual methane fluxes from an organic soil in eastern Finland, originally drained and planted with birch ( Betula pendula ) and then later cultivated, were studied for two years using a chamber technique. The agricultural soils growing grass or barley or without vegetation, generally acted as sinks for CH₄. Surprisingly, the agricultural soils emitted CH₄ during a warm dry summer. The CH₄ oxidation capacity and CH₄ uptake rate of the forested site was three times that of agricultural soils. Also, the forest soil better retained its capacity to take up CH₄ during a dry summer. Despite periods of CH₄ emission, the agricultural soils were annual sinks for CH₄, with uptake rate of CH₄-C varying from 0.1 to 3.7 kg ha⁻¹ yr⁻¹. The forested soil had a methane uptake rate of 3.9 kg CH₄-C ha⁻¹ yr⁻¹. All the soils acted as sinks for CH₄ during winter, which contributed up to half of the annual CH₄ uptake. The capacity of soils to transport gases did not explain the larger CH₄ uptake rate in the forest soil. At the same gas filled porosity, the forest soil had a much larger CH₄ uptake rate than the agricultural soil. Neither the soil acidity (pH 4.5 and 6.0) nor high ammonium content appeared to limit CH₄ uptake. The results suggest that CH₄ oxidation in agricultural organic soil is more sensitive to soil drying than CH₄ oxidation in forested organic soil.     

Causes of soil acidification: a summary

Abstract. A review of recent data shows that (i) dissolved CO₂ has its greatest acidifying effect in soils with pH values above about 6.5, (ii) fertilizers containing NH₋₁⁺ ions or urea will acidify soil whether the ions are taken up directly by plants or are first nitrified, (iii) oxidation of nitrogen and sulphur in soil organic matter causes acidification especially after deforestation, and (iv) the acidifying effect of rainfall and dry deposition is due to sulphuric and nitric acids, SO₂ and NH₋₁⁺ ions. A table is given showing the order of magnitude of each source of acidification.     

Simple phosphorus saturation index to estimate risk of dissolved P in runoff from arable soils

Abstract. There is increasing evidence that phosphorus has been accumulating in the surface horizons of agricultural soils to the extent that some soils represent a potential diffuse source of pollution to surface waters. The relationships between equilibrium phosphorus concentration at zero sorption (EPC ₀) of soil and a number of soil physicochemical variables were investigated in the surface layers of arable and grassland agricultural soils sampled from the Thame catchment, England. Soil EPC₀ could be predicted from an equation including soil test (Olsen) P, soil phosphate sorption index (PSI) and organic matter content (OM) (R²=0.88; P <0.001) across a range of soil types and land use. The simple index Olsen P/PSI was found to be a good predictor of EPC₀ (R²=0.77; P <0.001) and readily desorbable (0.02 m KCl extractable) P (R²=0.73; P <0.001) across a range of soil types under arable having soil organic matter contents of <10%.     

Geologic Nitrogen as a Source of Soil Acidity

The origin of highly acidic (pH<4.5) barren soils in the Klamath Mountains of northern California was examined. Soil parent material was mica schist that contained an average of 2,700 mg N kg⁻¹, which corresponds to 7.1 Mg N ha⁻¹ contained in a 10-cm thickness of bedrock. In situ soil solutions were dominated by H⁺, labile-monomeric Al³⁺ and NO₃⁻, indicating that the barren area soils were nitrogen saturated—more mineral nitrogen available than required by biota. Leaching of excess NO₃⁻ has resulted in removal of nutrient cations and soil acidification. Nitrogen release rates from organic matter free soil ranged from 0.0163 to 0.0321 mg N kg⁻¹ d⁻¹. Nitrogen release rate from fresh ground rock was 0.0465 mg N kg⁻¹ d⁻¹. This study demonstrates that geologic nitrogen may represent a large and reactive nitrogen pool that can contribute significantly to soil acidification.     

Methane oxidation kinetics differ in European beech and Norway spruce soils

Coniferous forest soils often consume less of the greenhouse gas methane (CH₄) than deciduous forest soils. The reasons for this phenomenon have not been resolved. It might be caused by differences in the diffusive flux of CH₄ through the organic layer, pH or different concentrations of potentially inhibitory compounds. Soil samples were investigated from three adjacent European beech ( Fagus sylvatica ) and Norway spruce ( Picea abies ) stands in Germany. Maximal CH₄ oxidation velocities (V_max(app)) and Michaelis Menten constants (K_M(app)), retrieved from intact soil cores at constant CH₄ concentrations, temperature and matric potential, were twice as great in beech as in spruce soils. Also atmospheric CH₄ oxidation rates measured in homogenized soil samples displayed the same trend. Greatest atmospheric CH₄ oxidation rates were detected in the O_a horizon or in the upper 5 cm of the mineral soil. In contrast to the beech soils, the O_a horizon of the spruce soils consumed no CH₄. A differential effect due to divergent diffusive flux through the litter layer was not found. pH and ammonium concentration were similar in samples from both forest soil types. Ethylene accumulation in all soils was negligible under oxic conditions. These collective results suggest that the different atmospheric CH₄ uptake by beech and spruce soils is caused by different CH₄ oxidizing capacities of methanotrophic communities in the O_a horizon and top mineral soil.     

Acid rain, cation dissolution, and sulphate retention in three tropical soils

Surface and subsurface samples of three tropical soils were examined with respect to their interaction with dilute solutions of sulphuric acid of pH 3. In calcareous clayey samples with a large cation exchange capacity the H⁺ was replaced by an equivalent concentration of metal cations which remained in solution along with SO^2-₄ as counterion. In a coarse-textured neutral soil with small cation exchange capacity, there was less chemical interaction and a major proportion of the H₂SO₄ remained unchanged in the equilibrium solution. Another soil exhibited considerable ability to remove SO^2-₄ from solution and, therefore, the total ionic concentration was greatly reduced. Other samples showed behaviour which was intermediate to these three types.
The ability to adsorb SO^2-₄ is one of the most important factors which determines the nature of the interaction of soil with dilute sulphuric acid. This ability was shown to be affected by the content of hydrous sesquioxides and organic matter in these soils.     

Soil S availability in upland pastures of NE Scotland: relationship of extractable soil S and soil respiration to soil and site characteristics

Abstract. The mean extractable sulphur (S) concentration in 315 upland topsoil samples collected in 1988/89 from beneath pasture in NE Scotland was 13 μg S g⁻¹ (range 2–77 μg S g⁻¹). More than two thirds of the samples had S concentrations less than that acceptable for productive soils. Continued decreases in atmospheric S inputs may have increased this proportion subsequently. The analysis of herbage S also indicated that two-thirds of the samples were below 0.2% S. A 'respirometric index', namely CO₂ produced during cellulose decomposition without added S as a percentage of that produced with added S, was significantly less than 100% in a quarter of the soils. Results of three different extraction procedures suggested that sulphate in the soils was present mainly as free plus adsorbed rather than precipitated forms. Soil extraction identified a significant non-sulphate S fraction, presumably organic S. The variability in extractable S stemmed from a combination of geographical, depositional and local site and soil factors. Extractable S was significantly correlated with soil organic matter content and inversely with soil pH and together these factors explained 37% of the variability. While significant differences in mean concentrations between geographical area, soil association and drainage status were evident, no trends could be observed between the major soil subgroups or with altitude.     

Methane consumption in a frequently nitrogen-fertilized and limed spruce forest soil after clear-cutting

Abstract. The effects of especially frequent nitrogen (N) additions (from 1959 to 1986, totalling 860 kg N ha⁻¹) and liming (in 1958 and 1980, totalling 6000 kg CaCO₃ ha⁻¹) on CH₄ uptake by a boreal forest soil were studied in a stand of Norway spruce. Except for a forested reference plot, the stand was clear-cut in January 1993 and the following year one-half of each clear-cut plot was prepared by mounding. Fluxes of CH₄ were measured with static chambers in the autumn before clear-cutting and during the following four summers. The average CH₄ uptake during 1993–96 in the forested reference plot was 82 μg CH₄ m⁻² h⁻¹(ranging from 10 to 147 units). In the first summer after clear-cutting, the cleared plot showed 42% lower CH₄ uptake rate than the forested reference plot, but thereafter the difference became less pronounced. The short-term decrease in CH₄ consumption after clear-cutting was associated with increases in soil NH₄⁺ and NO₃⁻concentrations. Mounding tended at first to stimulate CH₄ uptake but later to inhibit it. Neither liming nor N-fertilization had significant effects on CH₄ consumption. Our results suggest that over the long term, in N-limited upland boreal forest soils, N addition does not decrease CH₄ uptake by the soil.     

Effects of sorbed orthophosphate on zinc status in three soils of eastern Canada

P-Zn interactions can affect fertilizer use and produce Zn deficiencies with certain crops. Phosphorus-Zn sorption-desorption reactions were studied in topsoil and subsoil samples from three Quebec soils. Soils were equilibrated with P solutions, then with Zn solutions, and finally with solutions containing no P or Zn. The first equilibration evaluated P sorption (P_s), the second evaluated Zn sorption (Zn_s) after P sorption (P_s), and the third evaluated Zn desorption (Zn_D) as related to added P. Subsequently, Zn fractions were extracted sequentially with KNO₃ (Zn _{kno 3}), NaOH (Zn_NaOH) solutions and concentrated HN0₃+ H₂0₂(Zn_HNO,).
One mmole sorbed P resulted in increases of 0.5 to 1.0 meq (mean = 0.72) increases in cation exchange capacity (CEC). Increased Zn_s with added P was equivalent to 4 to 5% of the increase in CEC induced by P_s in the Uplands (sand) and St. Bernard (loam) soils, and 0.4 to 0.9% in the Dalhousie (clay) soils, while one meq increase in CEC resulted in 1.5-3.5% decrease in Zn_D. There existed positive correlations between P_s and extractable soil Fe materials. Phosphate sorption enhanced associations between Zn_s, Zn_D or Zn fractions and soil organic or crystalline Fe contents, confirming that P addition increased specific sorption of Zn on Fe components. Other mechanisms including precipitation, P-induced negative charge and 'bridge' effects are also discussed.     

Kinetic Parameters of Gross N Mineralization of Peat Soils as Related to the Composition of Soil Organic Matter

Peat land has been considered as an alternative type of land for agricultural development especially in the tropics. In the present study, the N-supplying capacity, one of the most important soil properties in terms of crop production, of peat soils was examined. Ten peat soil samples were collected from Indonesia, Malaysia, and Japan. Gross N mineralization in the soil samples was estimated using a zero-order model, and kinetic parameters of mineralization were determined using a simple type model. Soil organic matter composition was investigated using ¹³C CPMAS NMR. Mineralization potential ( N ₀), apparent activation energy ( E _a), and mineralization rate constant ( k ) ranged between 571–2,445 mg kg⁻¹, 281–8,181 J mol⁻¹, and 0.009–0.020 d⁻¹, respectively. Although none of the parameters showed a significant correlation with the soil C/N ratio, a negative correlation was observed between the k value and the ratio of the proportion of alkyl C in total C to that of O -alkyl C estimated by ¹³C CPMAS NMR. The latter suggested that the k values were higher in the peat soils relatively rich in readily decomposable organic matter including carbohydrates.     

Aluminium solubility related to secondary solid phases in upper B horizons with spodic characteristics

We examined the aluminium solubility in the upper B horizon of podzols and its relation to the solid phase of the soil in 60 samples covering a pH range from 3.8 to 5.1. Solid phases were characterized by extractions with acid oxalate and pyrophosphate (pH 10). The solubility of Al was studied in a batch experiment in which samples were equilibrated with 1 m m NaCl at 8°C for 5 days. We also monitored the dissolution kinetics of Al and Si, in some samples. The oxalate and pyrophosphate extractions suggested that secondary Al was mainly organically bound in most soils, and imogolite-type materials seemed to constitute much of inorganic secondary Al. No single gibbsite or imogolite equilibrium could explain Al³⁺ activities. In all samples Al solubility, defined as log{Al³⁺} + 1.65pH, was closely related to the molar ratio of aluminium to carbon in the pyrophosphate extracts (Al_p/C_p). Solubility increased with the Al_p/C_p ratio until the latter reached ≈ 0.1. This indicated that solubility was controlled by organic complexation, at least when Al_p/C_p was small. Silica dissolved slowly in most soils used in the kinetic experiments. We conclude that imogolite-type materials in the upper B horizon dissolved slowly because of coating with humic substances or ageing or both.     

Effects of organic matter and calcium on soil structural stability

The cationic bridging effect of the calcium ion (Ca²⁺) and the flocculating ability of clay and organic matter are crucial in the formation and stability of soil aggregates. They are therefore likely to influence the soil's saturated hydraulic conductivity ( K _s). We tested the individual effects of these factors on aggregate stability and related hydraulic properties, and studied the influence of clay mineralogy also. Samples from the surface (0–10 cm) of three contrasting soils in Trinidad were used. The soils were treated with three levels of Ca²⁺ and three levels of organic matter in a 3 × 3 × 3 factorial design and incubated for 14 days. Both aggregate stability and saturated hydraulic conductivity were influenced by all factor combinations. Interactions between soil type and Ca²⁺ revealed the importance of polyvalent cations in aggregate stability of soils with low activity minerals. The influence of organic matter varied with quantity; the more there was, the more stable the soil became, particularly in the soil containing little clay. Clay dispersion and slaking of expanding minerals occurred even with large additions of Ca²⁺ and organic matter, emphasizing the overall influence of mineralogy in determining the response of soils to stability treatments.     

Metal-humus complexes in A horizons of Thai and Korean red and yellow soils

Carbon, Al and Fe (C_pyr, Al_pyr and Fe_pyr) were extracted with 0.1 m Na₄P₂O₇ from 26 A horizon samples of tropical Thai and temperate Korean soils (Ultisols, Alfisols, Oxisols and Inceptisols). The soils, except for one Thai Inceptisol, had similar total C (0.35–3.29%) and C_pyr/total C ratios (0.20–0.41). There were approximately linear relationships between total C or C_pyr and clay content; two groups of soils gave different linear relationships. A curvilinear relationship between C_pyr and (Al + Fe)_pyr (milli-atom kg⁻¹) that can be approximated by an equation: C_pyr= 53 (Al_pyr+ Fe_pyr)^1/2– 24 was also found for most Thai and Korean soils. The above relationships indicated that total C and C_pyr would be close to zero at zero clay or zero (Al + Fe)_pyr. It was inferred that clay-humus interaction has a primary importance in the determination of humus content in red and yellow soils in tropical and temperate regions and that the main role of clay is to supply Al and Fe that complex and stabilize humus against microbial degradation.     

Assessing Passive Capillary-Wick Samplers for monitoring resident nitrate concentration in real field

Abstract. We evaluated the effectiveness of capillary-wick samplers (PCAPS) for continuous monitoring of resident nitrate concentration in three 'soil-crop-climate' systems differing in soil type, land use and climate. These systems involved: (i) acid silty soils under a beech-oak forest affected by heavy N-NH₄⁺ deposition in Belgium; (ii) silty soils under wheat cropping and a short rotation willow coppice plantation (SRC) in Belgium; and (iii) volcanic ash soils under plantain cultivation with and without urea fertilization in Colombia. The PCAPS continuously applied a suction of 0 to 5.4 kPa to the soil water below the effective rooting zone without the need for an auxiliary vacuum source. The nitrate concentrations showed large variations over time and ranged between 6–192 mg l^–1 under forest, 19–143 mg l^–1 under wheat, 11–47 mg l^–1 under SRC and 3–138 mg l^–1 under fertilized plantain. The analysis of the soil leachates collected with PCAPS confirms previous results dealing with leaching of nitrate and alkaline and alkaline-earth cations in similar 'soil-crop-climate' systems. It was concluded that PCAPS was a suitable tool to collect soil solutions and that it could help to assess nitrate leaching losses in various ecological or cropping conditions.     

Ochre deposits and associated bacteria in some field drains in Scotland

Drainage systems installed in highly organic soils, with the ground water pH near to neutral and affected by iron ochre deposition, were studied at six sites in Scotland. The iron ochre was composed of iron–encrusted filamentous bacteria entrapping amorphous material. The filamentous bacteria were identified as Gallionella spp., Sphaerotilus natans, Leptothrix ochracea and Leptothrix pseudo–ochracea . Heterotrophic iron–oxidizing bacteria were also present in the iron ochre, but Thiobacillus ferroxidans was not isolated. Differences in the proportion of species could not be correlated with peat type or the age of the drainage system. Sulphate reducing bacteria and bacteria capable of reducing Fe³⁺ were also isolated from the drainage water.
A preliminary investigation of the importance of biotic factors in the kinetics of Fe²⁺ oxidation showed that the rate of Fe²⁺ oxidation in unfiltered drainage water, was about twice the rate in filtered water, at 10°C. Incubations over a range of temperatures showed that the differences in rates were greatest at low temperatures.     

Factors affecting formation of methyl nitrite in soils

The formation of CH₃ONO in 11 soils treated with HNO₂ or NaNO₂ in a closed system, was studied by measuring the concentration in the gas space above the soil and by absorbing CH₃ONO in HI. The gaseous concentration of CH₃ONO increased and then decreased following additions of HNO₂ or NaNO₂, and the production of CH₃ONO increased with increasing concentrations of HNO₂ or NaNO₂ added to soils.
The amounts of CH₃ONO trapped in HI were 13.5 to 20.4 times higher than those determined by integrating under the net production curves. The evolved CH₃ONO amounted to 0.4 to 3.5% of added NO₂⁻, and 4.2 to 50% of the gaseous forms of N absorbed by acidic KMnO₄ solution. The CH₃ONO evolved from soils was positively correlated with the methoxy content of the soils, and inversely related to soil pH, with negligible amounts being evolved from alkaline soils. The results show that CH₃ONO is a product of NO₂⁻ decomposition in soils, and indicate that small concentrations of the gas may be produced in N–fertilized soils in which NO₂⁻ accumulates.     

Interaction forces between soil particles: shear moduli of the < 2 μm size fraction

The shear moduli of the < 2 μm size fraction of three soils have been measured for samples of 10–35% w/w solid. Samples were thixotropic, the shear modulus increasing with time. This increase can be described by simple models and visualized in terms of the formation of links between particles. For a given soil the shear modulus increases with ionic form in the order Ca²⁺∼Mg²⁺ + + < Li⁺ and varies in a complex manner with electrolyte concentration.     

Inhibition of nitrate accumulation in tropical grassland soils: effect of nitrogen fertilization and soil disturbance

In acid soils in the Eastern Plains of Colombia, forage grasses planted on land prepared before the previous dry season produced 40–50% more dry matter than when land was prepared immediately before planting. Virtually no NO₃⁻ accumulated in surface (0–10 cm) soil from three native undisturbed savanna sites. Where land was ploughed before the dry season, NO₃⁻ levels increased gradually after a 2–3 month lag, and dropped at the beginning of the rains. In samples incubated for 4 weeks, more NO₃⁻ accumulated in the wet than the dry season. A similar 2–3-month lag occurred when land was ploughed after the dry season. NH₄⁺ levels were higher in ploughed than savanna soils, and rose in all soils at the beginning of the rains. More NO₃⁻ and NH₄⁺ accumulated on incubation in pots than in soil cores. Forage grasses inhibited NO₃⁻ accumulation in the soil, relative to plant-free plots, and legumes stimulated it. N fertilization overcame this inhibition except in the case of Brachiaria humidicola .     

Errors in the estimation of soil water properties and their propagation through a hydrological model

Abstract. The Agricultural Catchments Research Unit model (ACRU) includes a decision support system (DSS) for estimating the water content of soil at field capacity (θ _fc ) and wilting point (θ _wp ) when these characteristics are not directly measurable. Three methods of estimation are proposed: (a) based on silt and clay content and bulk density, (b) based on clay content only, and (c) based on soil series. These three pedotransfer functions are compared with respect to both the estimation of θ _fc and θ _wp and the propagation of errors when the actual evapotranspiration of a wheat crop (E) is predicted over the growing season by the ACRU model.
The standard error of estimation was between 0.066 and 0.082 m³/m³ for θ _fc , between 0.056 and 0.069 m³/m³ for θ _wp and between 29.9 and 34.8 mm of water for E. The method based on silt and clay contents and bulk density predicted θ _fc and θ _wp for non-swelling soils most precisely. The method based on soil series was better than other methods for swelling soils. It also performed better for estimating available water capacity and consequently for predicting E from a conceptual soil water model. The propagated error of estimating θ _fc and θ _wp using the DSS reached 15–18% of the simulated E. The error in the prediction of E can reach 26–30% when spatial variation in soil properties is also estimated.     

Aluminium speciation and pH of an acid soil in the presence of fluoride

The aim was to determine whether the addition of F to an acid soil reduces the concentration of free Al³⁺ and other forms that have been shown to be toxic to plants. The ability of two different extracts to reflect Al speciation in the soil solution was also investigated. Addition of F (0-5.2μmolg⁻¹) to an acid soil (pH 4.15, soil solution) increased the pH and total concentrations of Al and F in the soil solution whereas Al³⁺ remained constant or decreased. Soil solution pH, total soluble Al and Al extracted by 0.01 m CaCl₂ are not good predictors of the likelihood of aluminium toxicity in soils containing soluble fluoride.